Objective To explore the effects of bone marrow mesenchymal stem cells ( BMMSCs) on pulmonary fibroblasts of patients with nonspecific interstitial pneumonitis ( NSIP) , and investigate the therapeutic mechanism of BMMSCs for interstitial pulmonary fibrosis. Methods Human BMMSCs, human pulmonry fibroblasts ( HPFs) from NSIP patients, and normal HPFs were primary cultured in vitro. Then HPFs fromNSIP patients were co-cultured with BMMSCs or normal HPFs using Transwell co-culture system. After 24 hours, levels of transforming growth factor β1 (TGF-β1) and interferon inducible protein 10 ( IP-10) in culture supernatants were detected by ELISA method. Meanwhile, interleukin-6 ( IL-6) , IL-8, and monocyte chemotactic protein-1 ( MCP-1) in co-culture supernatants were detected by liquid chip. After co-cultured for 48 hours, total protein of HPFs was extracted and the expression level of alpha smooth muscle actin ( α-SMA) secreted by HPFs were detected by Western blot.Results HPFs from NSIP patients secreted higher level of IL-6, IL-8, and MCP-1 than normal HPFs, and secreted high level of α-SMA. In the Transwell co-culture system, human BMMSCs significantly reduced the levels of IL-6, IL-8, and MCP-1 secreted from HPFs of NSIP patients, and reduced the high expression of α-SMA in HPFs of NSIP patients. Conclusion Human BMMSCs can significantly reduce the secretion of IL-6, IL-8, MCP-1, and the expression of α-SMA in HPFs from NSIP patients.